无合适where条件过滤时尽量选择order by后的字段以驱动表进行查询

后台查询语句SELECTo.orders_id,s.orders_status_name,ot.text,af.affiliate_idFROMordersoLEFTJOINorders_totalotON(o.orders_id=ot.orders_id)LEFTJOINaffilia

后台查询语句

SELECT o.orders_id, s.orders_status_name, ot.text ,af.affiliate_id FROM orders o LEFT JOIN orders_total ot ON (o.orders_id = ot.orders_id) LEFT JOIN affiliate_sales AS afs ON afs.affiliate_orders_id = o.orders_id LEFT JOIN affiliate_affiliate AS af ON af.affiliate_id = afs.affiliate_id LEFT JOIN orders_status s ON o.orders_status = s.orders_status_id WHERE s.language_id = '1' AND (ot.class = 'ot_total' OR ot.orders_total_id IS NULL) ORDER BY o.orders_id DESC LIMIT 0, 20

有客户反应某后台查询非常慢,通过程序找到对应的sql，如上！

explain发现

+----+-------------+-------+--------+----------------------------+----------------------------+---------+-----------------------------+-------+----------------------------------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+--------+----------------------------+----------------------------+---------+-----------------------------+-------+----------------------------------------------+ | 1 | SIMPLE | s | ALL | PRIMARY | NULL | NULL | NULL | 21 | Using where; Using temporary; Using filesort | | 1 | SIMPLE | o | ref | orders_status | orders_status | 4 | banggood.s.orders_status_id | 31747 | | | 1 | SIMPLE | ot | ref | idx_orders_total_orders_id | idx_orders_total_orders_id | 4 | banggood.o.orders_id | 19 | Using where | | 1 | SIMPLE | afs | ref | PRIMARY | PRIMARY | 4 | banggood.o.orders_id | 11 | Using index | | 1 | SIMPLE | af | eq_ref | PRIMARY | PRIMARY | 4 | banggood.afs.affiliate_id | 1 | Using index | +----+-------------+-------+--------+----------------------------+----------------------------+---------+-----------------------------+-------+----------------------------------------------+

s表被作为驱动表，s表为全表扫描,o表使用了status类型的可选择性非常低的字段作为索引。

初步一看就知道索引使用不恰当！

我们可以看到这条语句where条件中，没有什么合适的可驱动条件；但是，在order by中，发现order by o.orders_id(orders_id为orders表的主键)。我们就可以利用这个特性！

强制使用orders表的orders_id索引进行驱动！

更改如下：

EXPLAIN SELECT o.orders_id, s.orders_status_name, ot.text ,af.affiliate_id FROM orders o FORCE INDEX(PRIMARY) LEFT JOIN orders_total ot ON (o.orders_id = ot.orders_id) LEFT JOIN affiliate_sales AS afs ON afs.affiliate_orders_id = o.orders_id LEFT JOIN affiliate_affiliate AS af ON af.affiliate_id = afs.affiliate_id LEFT JOIN orders_status s ON o.orders_status = s.orders_status_id WHERE s.language_id = '1' AND (ot.class = 'ot_total' OR ot.orders_total_id IS NULL) ORDER BY o.orders_id DESC LIMIT 0, 20; +----+-------------+-------+--------+----------------------------+----------------------------+---------+--------------------------------+------+-------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+--------+----------------------------+----------------------------+---------+--------------------------------+------+-------------+ | 1 | SIMPLE | o | index | NULL | PRIMARY | 4 | NULL | 1 | | | 1 | SIMPLE | s | eq_ref | PRIMARY | PRIMARY | 8 | banggood.o.orders_status,const | 1 | Using where | | 1 | SIMPLE | ot | ref | idx_orders_total_orders_id | idx_orders_total_orders_id | 4 | banggood.o.orders_id | 19 | Using where | | 1 | SIMPLE | afs | ref | PRIMARY | PRIMARY | 4 | banggood.o.orders_id | 11 | Using index | | 1 | SIMPLE | af | eq_ref | PRIMARY | PRIMARY | 4 | banggood.afs.affiliate_id | 1 | Using index | +----+-------------+-------+--------+----------------------------+----------------------------+---------+--------------------------------+------+-------------+

对比两次profiling；

前者：

+--------------------------------+------------+-----------+------------+--------------+---------------+ | Status | Duration | CPU_user | CPU_system | Block_ops_in | Block_ops_out | +--------------------------------+------------+-----------+------------+--------------+---------------+ | starting | 0.000027 | 0.000000 | 0.000000 | 0 | 0 | | Waiting for query cache lock | 0.000006 | 0.000000 | 0.000000 | 0 | 0 | | checking query cache for query | 0.000130 | 0.000000 | 0.000000 | 0 | 0 | | checking permissions | 0.000007 | 0.000000 | 0.000000 | 0 | 0 | | checking permissions | 0.000003 | 0.000000 | 0.000000 | 0 | 0 | | checking permissions | 0.000003 | 0.000000 | 0.000000 | 0 | 0 | | checking permissions | 0.000003 | 0.000000 | 0.000000 | 0 | 0 | | checking permissions | 0.000007 | 0.000000 | 0.000000 | 0 | 0 | | Opening tables | 0.000130 | 0.000000 | 0.000000 | 0 | 8 | | System lock | 0.000017 | 0.000000 | 0.000000 | 0 | 0 | | Waiting for query cache lock | 0.000033 | 0.000000 | 0.000000 | 0 | 0 | | init | 0.000057 | 0.000000 | 0.000000 | 0 | 0 | | optimizing | 0.000026 | 0.000000 | 0.000000 | 0 | 0 | | statistics | 0.000041 | 0.000000 | 0.000000 | 0 | 0 | | preparing | 0.000031 | 0.000000 | 0.000000 | 0 | 0 | | Creating tmp table | 0.000111 | 0.001000 | 0.000000 | 0 | 0 | | executing | 0.000007 | 0.000000 | 0.000000 | 0 | 0 | | Copying to tmp table | 3.541123 | 0.968852 | 2.357642 | 75800 | 0 | | converting HEAP to MyISAM | 0.239566 | 0.038994 | 0.198969 | 0 | 262152 | | Copying to tmp table on disk | 174.185144 | 13.864893 | 35.361625 | 2135152 | 2500280 | | Sorting result | 20.923419 | 0.127980 | 3.017541 | 2770408 | 27536 | | Sending data | 0.045078 | 0.000000 | 0.002999 | 1208 | 0 | | end | 0.000018 | 0.000000 | 0.000000 | 0 | 0 | | removing tmp table | 0.881884 | 0.018997 | 0.160976 | 760 | 8 | | end | 0.003960 | 0.000000 | 0.002000 | 448 | 0 | | query end | 0.000012 | 0.000000 | 0.000000 | 0 | 0 | | closing tables | 0.031745 | 0.000000 | 0.000999 | 936 | 0 | | freeing items | 0.015499 | 0.000000 | 0.003000 | 808 | 0 | | Waiting for query cache lock | 0.000017 | 0.000000 | 0.000000 | 0 | 0 | | freeing items | 0.000791 | 0.000000 | 0.000000 | 0 | 0 | | Waiting for query cache lock | 0.000009 | 0.000000 | 0.000000 | 0 | 0 | | freeing items | 0.000003 | 0.000000 | 0.000000 | 0 | 0 | | storing result in query cache | 0.000009 | 0.000000 | 0.000000 | 0 | 0 | | logging slow query | 0.000003 | 0.000000 | 0.000000 | 0 | 0 | | logging slow query | 0.000010 | 0.000000 | 0.000000 | 0 | 0 | | cleaning up | 0.000007 | 0.000000 | 0.000000 | 0 | 0 | +--------------------------------+------------+-----------+------------+--------------+---------------+

各种cpu，io损耗，惨不忍睹！其中最大的消耗是Copying to tmp table on disk。